The SIR provided [4'-44% 17 O]Tyrosine; 1.3g We plan to do pulsed ENDOR studies on 17) labelled tyrosine radicals as part of our investigations into how a Photosystem H tyrosine radical together with the Mn cluster of PSH oxidize water. The first efforts will be on UV-generated tyrosine radicals. We plan to use pulsed ENDOR to dtudy motion of such radicals by examining the ENDOR lineshapes as a function of temperature. Similar studies will then be performed on the 170 tyrosine incorporated into PSII